1. Field of the Invention
The present invention relates to a process for preparing a superconducting thin film of bismuth-containing compound oxide. More particularly, it relates to an improved process for depositing on a substrate a bismuth (Bi) type superconducting thin film possessing an improved high critical current density (Jc) as well as a high critical temperature (Tc).
2. Description of the Related Art
Superconductivity is a phenomenon which is explained as a kind of phase change of electrons under which the electric resistance becomes zero and perfect diamagnetism is observed.
Several superconducting devices have been proposed and developed in electronics, which is a typical field to which the superconducting phenomenon is applicable. A typical application of a superconductor is Josephson device, in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to a smaller energy gap of the superconducting material. It is expected that the Josephson device will be utilized as a high-sensitivity sensor or detector for sensing very weak magnetic fields, microwaves, radiant rays or the like since variation of an electromagnetic wave or magnetic field is reflected in a variation of the Josephson effect and can be observed as a precise quantum phenomenon. Development of superconducting devices such as high-speed logic units or of no powerloss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption. The critical temperature "Tc" of superconductivity, however, could not exceed 23.2.degree. K. of Nb.sub.3 Ge, which was the the highest Tc for the past ten years.
The possibility of existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986, p189).
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses a Ba--Pb--Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba--Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10.degree. K. and hence usage of liquefied helium (boiling point of 4.2.degree. K.) as a cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4, which is called a K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides shows higher Tc such as 30.degree. K., which is extremely higher than known superconducting materials.
It was also reported in February 1987 that C. W. Chu et al. discovered, in the United States of America, another superconducting material, the so called YBaCuO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having a critical temperature of about 90.degree. K. (Physical Review Letters, Vol. 58, No. 9, p908).
An other type of new superconducting material is a compound oxide of Bi--Sr--Ca--Cu--O system reported by Maeda et al (Japanese Journal of Applied Physics, Vol. 27, No. 2, pp1209-1210). The bismuth-type compound oxides including this Bi--Sr--Ca--Cu--O system have such advantages that they are chemically much stabler and can be prepared with reduced cost because they contain no rare earth elements.
The above-mentioned new types of superconducting materials can be obtained by solid reaction in a bulk form of a sintered block obtained by sintering a powder mixture of oxides or carbonates of constituent metal elements. They can also be deposited on a substrate in the form of a thin film by physical vapor deposition (PVD) techniques such as RF sputtering, vacuum deposition, ion-plating or MBE or chemical vapor deposition (CVD) techniques such as thermal CVD, plasma CVD, photo-CVD or MOCVD. It is a usual practice to subject the resulting thin films to heat-treatment in an oxygen-containing atmosphere to adjust the oxygen deficiency in the crystal.
The present applicant already proposed several processes for preparing thin films of the high-Tc superconductor on a substrate in the following patent applications: U.S. patent application Ser. No. 152,714 filed on Feb. 2, 1988 (now abandoned), U.S. patent application Ser. No. 167,895 filed on Mar. 13, 1988 (now abandoned), U.S. patent application Ser. No. 195,145 filed on May 18, 1988 (now U.S. Pat. No. 4,900,716), U.S. patent application Ser. No. 195,147 filed on May 18, 1988 (now abandoned), U.S. patent application Ser. No. 200,206 filed on May 31, 1988 (now U.S. Pat. No. 4,996,185), U.S. patent application Ser. No. 286,860 filed on Dec. 20, 1988 (pending), U.S. patent application Ser. No. 289,718 filed on Dec. 25, 1988 (pending), U.S. patent application Ser. No. 289,719 filed on Dec. 25, 1988 (pending), and U.S. patent application Ser. No. 290,309 filed on Dec. 26, 1988. The present invention has been completed along the same line as these patent applications.
The present invention concerns the superconducting thin films of bismuth (Bi) type compound oxide. This type superconducting thin film has been prepared by deposition techniques such as RF sputtering, vacuum deposition or MO-CVD technique. They possess, however, very low critical current density (Jc) although they showed very high critical temperature (Tc), so that they were difficult to utilize in practical uses such as electronic devices or wiring materials.
An object of the present invention is to overcome the shortcomings of the conventional bismuth (Bi) type superconducting thin films and to provide an improved process for increasing the critical current density (Jc) thereof.